MINECON was over so I uploaded a new world and closed and re-opened the Realm and it's now up and running again.
Still not working!
Re-uploading the downloaded world has not helped.
I tried closing and re-opening and that did not help.
I managed to download the world, and it loads OK offline.
I have same issue in 1.1 on my Followers Realm.
Windows 10 constantly hitting 100% CPU usage. Local worlds with custom entities/resource packs.